1. Field of the Invention
The present invention relates generally to reference circuits, and more specifically to a bandgap reference circuit.
2. Description of the Related Art
A bandgap reference circuit is used to generate a precise and a stable output voltage. The generated voltage is independent of process, voltage, and temperature. The bandgap reference circuit is widely used in various analog and digital circuits that require a precise voltage for operation.
FIG. 1 illustrates one commonly used bandgap reference circuit 100. Referring to FIG. 1, the bandgap reference circuit 100 includes PMOS transistors M1, M2, and M3, an operational amplifier OP, resistors R1 and R2, and bipolar transistors Q1, Q2, and Q3. If the base current is neglected, the output voltage VOUT of the bandgap reference circuit 100 can be expressed as:
                    VOUT        =                              VEB            ⁢                                                  ⁢            3                    +                      VT            ×            ln            ⁢                                                  ⁢            N            ×                          (                                                R                  ⁢                                                                          ⁢                  2                                                  R                  ⁢                                                                          ⁢                  1                                            )                                                          (        1        )            
Where VEB3 is the emitter-base voltage of the bipolar transistor Q3, VT is the thermal voltage at room temperature, and N is the ratio of the emitter areas of the bipolar transistor Q2 to the emitter areas of the bipolar transistor Q1.
As can be seen from the equation (1), by adjusting the resistance ratio of resistor R2 to R1, the conventional bandgap reference circuit 100 can provide a stable reference voltage VOUT having a zero temperature coefficient. The voltage level of the voltage VOUT is at around 1.25V, which is approximately equal to the silicon energy gap measured in electron volts, i.e., the silicon bandgap voltage.
However, in order to meet the application requirement of different integrated circuits, a reference voltage with a substantially zero temperature coefficient at the lower voltage level is needed.